The Detection of Ionizing Radiation


Various instruments and devices are used for the detection of radiations. The best known device is the Geiger-Muller counter. This device consists of two parts, a detecting tube and a counter. The heart of the system is the detecting tube, which consists of a pair of electrodes surrounded by an ionizable gas. As radiation enters the tube, it ionizes the gas. The ions produced travel toward the electrodes, between which there is a high voltage. The ions cause pulses of current at the electrodes, which are picked up and recorded on the counter.

The Geiger tube is most sensitive to beta radiation. Gamma radiation can pass right through the tube without being counted, and some alpha radiation can't make it through the window of the tube. The Geiger-Muller counter indicates the counts per minute of radiation entering the tube, but it doesn't tell you the energy of the radiation.

A scintillation counter is a device that not only counts radioactivity, but also enables the operator to determine the energy of the radiation. The principle of operation involves the radiation reacting with a crystal containing sodium iodide and thallium iodide, which produces a series of flashes of varying intensity. The intensity of the flashes is proportional to the energy of the radiation.

Film badges are small portable devices that are worn by people such as x-ray technicians and nurses, who may be exposed to radiation. The badge contains a piece of photographic film that is removed monthly and developed. The darker the film badge, the greater the degree of exposure.

Another device called a dosimeter is quickly replacing the film badge. One type of dosimeter works on the property of thermoluminescence, and is called a TLD for short. The TLD consists of a penlike device and a reading unit. The penlike device, which is worn by the individual, contains a crystal such as lithium fluoride, which absorbs radiation. When the lithium fluoride crystal absorbs the radiation, its structure changes slightly. To determine the amount of radiation that the crystal has absorbed, the penlike device is placed in its reading unit, where it is heated quickly. This causes the lithium fluoride crystal to return to its original state.

As it does, it gives off visible light. The visible light is proportional to the radiation absorbed by the lithium fluoride crystal.


Copyright 1997 James R. Fromm (jfromm@3rd1000.com)